1. Field of the Invention
The present invention relates to a radio transmission unit having a control circuit for avoiding an interference from a first radio transmission system, provided in a second radio transmission system, which communicates with the first radio transmission system through the same frequency band, capable of avoiding a hidden terminal problem by exchanging a transmission request signal (RTS) and reception preparation completion signal (CTS), and a radio transmission system and a transmission timing adjusting device.
2. Description of the Related Art
2.4 GHz frequency band is a band permitted for transmission without license and currently, various transmission types have been standardized and made available. For example, IEEE802.11 series radio LAN and Bluetooth is available. Of them, radio LAN is one of very prevalent transmission types, which secures a relatively long transmission distance and a high transmission speed.
Some of various types of radio transmission systems considered in recent years require energy saving transmission method. As an example of the energy saving transmission systems, there is a radio transmission system described in Japanese Patent Laid-open Publication No. 09-162798 as a conventional technology. In this radio transmission system, one main station (described as base station in Japanese Patent Laid-open Publication No. 09-162798) communicates with plural subsidiary stations (described as mobile stations in Japanese Patent Laid-open Publication No. 09-162798). According to this system, the main station transmits a beacon signal periodically and after the subsidiary station receives the beacon signal, a predetermined period turns to a period which allows the subsidiary station to transmit and then it communicate with the main station in that period. The subsidiary station operates intermittently so that its transmission circuit activates only in the communication allowed period, synchronizing with the beacon signal so as to suppress consumption power.
In the 2.4 GHz band radio transmission system also, standardization about the energy saving radio transmission system using a method for operating the transmission circuit intermittently synchronous with the beacon signal transmitted periodically as described in the Japanese Patent Laid-open Publication No. 09-162798 as the conventional technology has been progressed. Further, according to this standard system, the transmission distance is suppressed to a shorter distance than the radio LAN in order to establish an energy saving transmission system.
In such 2.4 GHz band, plural different radio transmission systems can be used. Thus, if plural radio transmission systems using different radio transmission methods are used at the same place, there is such a problem that the transmission quality of each transmission system may deteriorate due to interference between those transmission systems.
According to the radio transmission system described in the Japanese Patent Laid-open Publication No. 2001-237720, to solve this problem, by measuring interfering electric power from a different transmission system and inhibiting transmission from a self terminal during receiving of the interfering wave, being interfered by other transmission system and interfering other transmission system are avoided.
However, according to the interference avoiding method described in the Japanese Patent Laid-open Publication No. 2001-237720, during transmission of a signal (interfering wave as viewed from self-transmission system) by other transmission system, transmission of the self-transmission system must be inhibited. That is, a period in which the transmission of the self-transmission system is inhibited is determined depending on transmission timing of other transmission system. For the reason, in a system based on such a method in which the transmission circuit is activated intermittently synchronous with the beacon signal, if the main station is receiving an interfering signal from other system at a time when it transmits the beacon signal, it becomes unable to transmit the beacon signal at a right timing.
Although interference to the self-transmission system can be avoided by executing the interference avoiding method proposed in the Japanese Patent Laid-open Publication No. 2001-237720 on the side of other transmission system to detect the beacon signal transmitted by the self-transmission system while inhibiting transmission during this period, if a transmission distance of the self-transmission system is shorter than the transmission distance of other transmission system, a signal of the self-transmission system reaches only part of terminals which interfere the self-transmission system within the other transmission system. As a result, interference from the other transmission system cannot be avoided completely.